The present invention relates to an ink jet recording apparatus capable of ejecting extremely small ink droplets and a method of driving an ink jet recording head incorporated in the apparatus.
An ink jet recording apparatus includes a recording head having a multiplicity of nozzle orifices arranged in a sub-scanning direction (a recording paper feeding direction) and is arranged to attain desired printing result by moving the recording head in a main-scanning direction (a width direction of the recording sheet) by a carriage mechanism to thereby perform predetermined paper feeding. Ink droplets are respectively ejected at predetermined timings from the respective nozzle orifices of the recording head based on dot pattern data which is obtained by converting print data inputted from a host computer. These ink droplets reach and attach to a print recording medium such as a recording sheet to thereby form dot images and complete the printing operation.
The recording head is configured in a manner that the deformation of a piezoelectric vibrator is transmitted to a vibration plate and a pressure chamber is contracted to increase the inner pressure thereof to thereby eject an ink droplet from the nozzle orifice. The piezoelectric vibrator is deformed by changing driving voltage inputted to the piezoelectric vibrator. In general, the piezoelectric vibrator is arranged so as to have larger deformation when higher driving voltage is inputted thereto. Thus, an ink droplet is ejected by applying a drive signal for changing the voltage level of the driving voltage to the piezoelectric vibrator to thereby expand and contract the pressure chamber.
As described above, the ink jet recording apparatus constitutes an image depending on whether ink droplets are ejected or not, that is, depending on the presence or non-presence of dot images. Thus, the ink jet recording apparatus can not print and output half-tone such as a gray image, if the apparatus is as it is.
Thus, there has been employed a method in which half-tone is realized by forming a single pixel with plural dots such as 4xc3x974, 8xc3x978 matrix. Although it is possible to perform finer tone reproduction when the pixel resolution is made higher, the substantial resolution rather degrades if the pixel resolution is made higher without changing the diameter of each recording dot. On the other hand, if each dot diameter is large, the graininess in a highlight image becomes remarkable. Thus, in order to perform tone reproduction with a high resolution, it is required to make the volume of an ink droplet as small as possible to thereby make the diameter of a recording dot small.
FIG. 7 shows a related drive signal for ejecting a fine ink droplet. This example of the signal is employed in such a type of recording head that a piezoelectric vibrator changes in a direction for expanding a pressure chamber when a driving voltage rises, while the piezoelectric vibrator changes in a direction for contracting the pressure chamber when the driving voltage lowers.
In a standby state P0 of the aforesaid drive signal, as shown in FIG. 8A, a meniscus 50 stops at a nozzle orifice 28. When a signal (P1) for rising the voltage from the minimum driving voltage VL in the standby state P0 to a maximum driving voltage VH1, the pressure chamber expands so that the meniscus 50 is pulled toward the pressure chamber from the nozzle orifice 28 as shown in FIG. 8B. Then, after holding the maximum driving voltage VH1 for a predetermined time period (P2), a signal (P3) for rapidly lowering the voltage to a voltage VH2 which is almost the middle between VL and VH1 is inputted, and the voltage VH2 is held for a predetermined time period (P4). At this time, the pressure chamber in the expanded state contracts to increase the pressure therein, whereby ink in the vicinity of the center of the meniscus 50 thus pulled is ejected and jetted as an ink droplet as shown in FIG. 8C. Thereafter, a signal (P5) for lowering the voltage to the minimum driving voltage VL same as that of the standby state at a relatively slow speed not ejecting an ink droplet is inputted, whereby the meniscus 50 is returned to the position of the nozzle orifice 28 as shown in FIG. 8D while the residual vibration thereof is damped.
In the recording apparatus using the drive signal, the pressure within the pressure chamber is increased in the state where the meniscus 50 is once pulled to a large extent within the chamber thereby to eject the ink in the vicinity of the center of the meniscus 50 thus pulled as an ink droplet. Thus, an ink droplet relatively small as compared with the diameter of the nozzle orifice 28 can be ejected.
Recently, in order to further improve the resolution, there has been desired a recording apparatus capable of ejecting a further fine ink droplet. However, in the aforesaid related recording apparatus, the reduction of the diameter of an ink droplet to be ejected is limited. It is considered to make an ink droplet to be ejected fine by reducing the diameter of the nozzle orifice 28. However, if the diameter of the nozzle orifice 28 is reduced, it becomes difficult to process the nozzle orifice 28, so that the cost of the apparatus rises and the accuracy of the apparatus likely degrades. Further, there arises a problem that the clogging may be severe that is caused when the ink in the vicinity of the nozzle orifice 28 dries during the suspension or the like of the apparatus and the recovery from the clogging is difficult. Thus, such a proposal can not be actually realized.
The invention has been made in view of the aforesaid circumstance of the prior art, and an object of the invention is to provide an ink jet recording apparatus and a method of driving an ink jet recording head incorporated in the apparatus, capable of ejecting extremely small ink droplets without reducing the diameter of a nozzle.
In order to achieve the above object, according to the present invention, there is provided an ink jet recording apparatus, comprising:
a recording head, provided with a pressure chamber communicated with a nozzle orifice from which an ink droplet is ejected, and a vibration plate which constitutes a part of the pressure chamber;
a pressure generating element, which deforms the vibration plate to vary a volume of the pressure chamber, and
a drive signal generator, which generates a drive signal for driving the pressure generating element, the drive signal including;
a first waveform component, which drives the pressure generating element so as to contract the pressure chamber, to push out a meniscus of ink from the nozzle orifice such an extent that an ink drop is not ejected therefrom;
a second waveform component, which follows the first waveform component and drives the pressure generating element so as to expand the pressure chamber to a first volume, to pull the meniscus toward the pressure chamber;
a third waveform component, which follows the second waveform component and drives the pressure generating element so as to contract the pressure chamber from the first volume to a second volume which is larger than an initial volume of the pressure chamber, and hold the contracted state to eject an ink droplet from the nozzle orifice; and
a fourth waveform component which follows the third waveform component and drives the pressure generating element so as to contract the pressure chamber such an extent that an ink droplet is not ejected from the nozzle orifice.
In this configuration, since the meniscus is once pushed out and then pulled toward the pressure chamber, a portion in the vicinity of the center of the meniscus is locally pulled by the second waveform component. Since the third waveform component is inputted in this state thereby to contract the pressure chamber, the ink at an extremely small area in the substantial center of the meniscus moves to the nozzle orifice and is ejected therefrom as an ink droplet. Thus, an extremely small ink droplet can be ejected without reducing the diameter of the nozzle orifice and so the printing with a high resolution can be realized. Further, the speed of the ink droplets being ejected rises and the accuracy of the impact points of the ink droplets can be improved.
Preferably, a potential of an initial end of the first waveform component is higher than a lowest potential of the drive signal, and has a positive value.
In this configuration, the lowest potential can be set at the ground potential so that the control is made easier.
Preferably, a potential of a termination end of the fourth waveform component and a potential of an initial end of the second waveform component are identical.
In this configuration, the residual vibration of the meniscus due to the ink ejection can be damped sufficiently. Thus, at the time of ejecting ink droplets in series, the next ejecting operation can be performed after sufficiently damping the residual vibration of the meniscus, so that the degree of the variation of the volumes of the ink droplets can be made small and so stable printing quality can be secured.
Here, it is preferable that the drive signal includes a fifth waveform component which follows the fourth waveform component and restores a potential of a termination end of the fourth waveform component to a potential which is identical with the initial end potential of the first waveform component.
In this configuration, it is not necessary to add an unnecessary signal for restoring the voltage at the time of generating the drive signals in series.
Preferably, a time period from an initial end of the first waveform component to an initial end of the second waveform component is identical with a time period obtained by multiplying a natural vibration period of the pressure chamber by an integer.
In this configuration, the generation of crosstalk can be suppressed so that ink droplets can be ejected more stably.
Alternatively, a time period from an initial end of the first waveform component to an initial end of the second waveform component is identical with a time period obtained by multiplying a natural vibration period of the vibration plate by an integer.
Also in this configuration, the generation of crosstalk can be suppressed so that ink droplets can be ejected more stably.
Preferably, a time period from a termination end of the fourth waveform component to a termination end of the fifth waveform component is identical with a time period obtained by multiplying a natural vibration period of the pressure chamber by an integer.
In this configuration, since a timing where the pressure chamber expands due to the fifth waveform component becomes almost opposite in the phase with respect to the residual vibration of a meniscus, the residual vibration of the meniscus can be damped more effectively. Thus, at the time of ejecting ink droplets in series, the next ejecting operation can be performed after sufficiently damping the residual vibration of the meniscus, so that the degree of the variation of the volumes of the ink droplets can be made small and so stable printing quality can be secured.
Preferably, a potential gradient of the first waveform component is variable in accordance with an environmental condition of the recording apparatus.
The viscosity of the ink or the like changes depending on the environmental condition such as temperature and humidity or the like in the periphery of the apparatus. In this configuration, even if the characteristics of the ink changes, a fine ink droplet can be ejected stably by optimally changing the potential gradient of the first waveform component in accordance with the environmental condition in the periphery of the apparatus. Incidentally, in the invention, xe2x80x9cenvironmental conditionxe2x80x9d refers to at least one of as temperature and humidity, for example, but not limited thereto.
Preferably, a potential difference between an initial end and a termination end of the first waveform component is 10% to 50% of a potential difference between an initial end and a termination end of the second waveform component.
In this configuration, sufficient ejecting speed of an ink droplet and stability thereof can be secured.
Preferably, the drive signal generator repetitively generates the drive signal at a predetermined times within a unit printing period.
In this configuration, the variable range of the diameter of a dot image is enlarged so that the multi-tone reproduction can be surely realized.
Here, it is preferable that at least one of the drive signals are selectively applied to the pressure generating element to form a single ink dot by at least one ink droplet.
In this configuration, since a plurality of different sizes of dot images are formed based on combination of a plurality of ink droplets, dot images with different sizes can be formed by using the one kind of the drive signal, so that the variable range of the diameter of a dot image is enlarged so that the multi-tone reproduction can be surely realized.
Preferably, the pressure generating element is an electromechanical transducer such as a plezoelectric vibrator.
According to the present invention, there is also provided a method of driving an ink jet recording head provided with a pressure chamber communicated with a nozzle orifice from which an ink droplet is ejected, and a vibration plate which constitutes a part of the pressure chamber, comprising the steps of:
a) contracting the pressure chamber from a first volume to a second volume so as to push out a meniscus of ink from the nozzle orifice such an extent that an ink drop is not ejected therefrom, and holding the contracted state;
b) expanding the pressure chamber from the second volume to a third volume so as to pull the pushed-out meniscus toward the pressure chamber;
c) contracting the pressure chamber from the third volume to a fourth volume, and holding the contracted state to eject an ink droplet from the nozzle orifice; and
d) contracting the pressure chamber from the fourth volume to a fifth volume such an extent that an ink droplet is not ejected from the nozzle orifice.
Preferably, the second volume and the fifth volume are identical.
Preferably, the driving further comprises the step of e) expanding the pressure chamber from the fifth volume to the first volume.
Here, it is preferable that the method further comprises the step of determining how many times the steps a)-e) are repeated within a unit printing period.
Further, it is preferable that the repeated number is determined in accordance with a size of ink dot to be formed.
Preferably, a duration of the step a) is identical with a time period obtained by multiplying a natural vibration period of the pressure chamber by an integer.
Alternatively, a duration of the step a) is identical with a time period obtained by multiplying a natural vibration period of the vibration plate by an integer.
Preferably, a duration of the step e) is identical with a time period obtained by multiplying a natural vibration period of the pressure chamber by an integer.
Preferably, a volume difference between the first volume and the second volume, and a duration of the step a) are determined in accordance with an environmental condition of the recording head.
Preferably, a volume difference between the first volume and the second volume is 10% to 50% of a volume difference between the second volume and the third volume.